Shutoff and control valves of the aforedescribed type, as will be apparent from the aforementioned patents and applications and German patent No. 3,724,100 and German open application DE-OS No. 37 31 754, can have a valve element and a valve seat with which that element cooperates and against which that element moves, which may be composed of an engineering ceramic, frequently referred to as a sinter ceramic, to minimize wear, especially where the fluid is an aggressive or corrosive medium, or entrains abrasive particles therewith.
It has been observed in practice, that a valve of this type frequently suffers a certain degree of adhesion of the movable valve element to the valve seat, such adhesion depending upon the fluid medium traversing the valve and being especially pronounced when the valve element is to be moved from a defined rest position into another defined position.
Indeed, and surprisingly, this adhesion is especially a problem when these parts in contact with one another have previously been lapped or honed to improve their surface finishes and hence the sealing effectiveness of the contact between the surfaces.
As a general matter, therefore, where the two ceramic parts must move relatively and have extremely low surface roughness and have been fabricated to extremely low tolerances with respect to roundness, planarity or the like, the adhesion is especially pronounced.
The source of this adhesion has not been well researched. The adhesion tends to damage valves, especially control valves, because it requires the application of relatively high valve-positioning forces to move the movable valve element and causes deterioration of servomotors and servotransmissions if they are not diminished to be able to supply the relatively large forces required. Of course, if the servomotors and transmissions are diminished to supply the requisite valve-shifting forces, relatively large servocontrol units may result, thereby increasing the capital cost of the system.
Since engineering ceramics are relatively brittle, the valve setting forces, on occasion, may be sufficient to rupture the ceramic elements.